Field
The disclosed concept pertains generally to nuclear fuel assemblies and more particularly to top nozzles for pressurized water nuclear reactors.
Background Information
In nuclear reactors of the type designed in the former Soviet Union, the reactor core is comprised of a large number of elongated fuel assemblies, each having a plurality of fuel rods held in an organized hexagonal array by a plurality of grids spaced longitudinally along the fuel rods and secured to stainless steel control rod guide thimbles. The stainless steel control rod guide thimbles extend above and below the ends of the fuel rods and are attached to the top and bottom nozzles, respectively. The fuel assemblies are arranged in the reactor vessel with the bottom nozzles resting on a lower core plate. An upper core plate rests on the top nozzles.
The top nozzles in the Soviet design are non-removably fixed to the stainless steel control rod guide thimbles of the fuel assembly. These complex nozzles perform several functions. First, they position the rod control cluster assembly (RCCA) relative to the guide tubes within the core so that the position of the RCCA relative to the upper core plate is fixed. The RCCA positions the control rods, which are inserted into the fuel assembly as a group or cluster.
The Soviet nozzle also dampens the velocity of the control rods using springs to remove energy when the RCCA rods are dropped into the reactor core during an emergency shutdown, commonly known as a “scram.” The nozzle also supplies spring loads for supporting the internals. When the upper core plate is lowered onto the nozzles, it compresses the nozzle spring. In addition, the Soviet nozzle is designed to protect the control rods when the fuel assembly is removed from the reactor vessel. Under these conditions, the RCCA is at or below the top edge of the nozzle. Finally, the Soviet design of the top nozzle allows the fuel assembly to be handled when lifted out of the core by transferring the loads through the nozzle.
In a VVER-1000 type Soviet designed reactor, the inventors have discovered problems with loading of fuel assemblies. More specifically, during refueling outages, the bottom nozzles of fuel assemblies being loaded into the reactor core make solid contact with the top nozzles of the fuel assemblies already loaded into the core. These interferences cause the fuel assemblies being loaded to be undesirably prevented from moving past the already loaded fuel assembly. This type of incident uses a significant amount of additional critical path time while loading the core. The result is extensions of outage time during fuel assembly loading that correspond to an increase in costs.
Accordingly, a new top nozzle and pressurized water nuclear reactor including a top nozzle are desired that will assure that engagements between the top nozzle and the bottom nozzle of a fuel assembly being loaded into the reactor core do not result in an undesirable extension of time during a refueling outage.